No matter how much time children and teens sit around, exercising improves their cardiometabolic risk profile, researchers found.

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This study found that across all levels of sedentary time, moderate-to-vigorous-intensity physical activity was associated with improvements in waist circumference, systolic blood pressure, triglycerides, HDL cholesterol, and insulin in participants ages 4 to 18.

The benefits of exercise were greater among those with the lowest level of sedentary time.

No matter how much time children and teens sit around, exercising improves their cardiometabolic risk profile, researchers found.

Across all levels of sedentary time, moderate-to-vigorous-intensity physical activity was associated with improvements in waist circumference, systolic blood pressure, triglycerides, HDL cholesterol, and insulin in participants ages 4 to 18 (P<0.05 for all), according to Ulf Ekelund, PhD, of Adenbrookes Hospital in Cambridge, England, and colleagues.

The benefits of exercise were greater, however, among those with the lowest level of sedentary time, the researchers reported in the Feb. 15 issue of the Journal of the American Medical Association.

"Children should be encouraged to increase their participation in physical activity of at least moderate intensity rather than reducing their overall sedentary time, as this appears more important in relation to cardiometabolic health," they wrote.

They noted, however, that reducing the amount of time young people spend in front of the TV -- a marker of sedentary time -- may still be an important goal because of TV viewing's association with other unhealthy behaviors and exposure to advertisements that may promote unhealthy diets.

Ekelund and colleagues pooled data from 14 studies that were included in the International Children's Accelerometry Database and reported physical activity and sedentary time objectively measured with an accelerometer. The studies included a total of 20,871 participants ages 4 to 18 from Australia, Brazil, Europe, and the U.S.

About three-quarters of the children and teens were normal weight, 18% were overweight, and 7% were obese.

The average time spent doing moderate-to-vigorous-intensity physical activity each day was 30 minutes, ranging from 12 minutes in the bottom third to 53 minutes in the top third. Average sedentary time was nearly six hours each day, ranging from 268 minutes in the bottom third to 433 minutes in the top third.

Time spent in at least moderate physical activity was significantly associated with improvements in all of the cardiometabolic risk factors assessed -- waist circumference, systolic blood pressure, triglycerides, HDL cholesterol, and insulin -- independent of sex, age, monitor wear time, sedentary time, and waist circumference (P<0.05 for all).

After accounting for physical activity levels, sedentary time was not associated with any outcome.

Participating in more moderate-to-vigorous-intensity exercise was associated with improved risk factors across all tertiles of sedentary behavior, although the differences between low levels and high levels of physical activity were greatest among participants with low sedentary time.

The average difference in waist circumference between the bottom and top tertiles of physical activity, for example, was 3.6 centimeters for those with high sedentary time and 5.6 centimeters for those with low sedentary time.

The researchers noted that if this pattern continues into adulthood, it could add up to substantial health risks for those with the largest waists. They pointed out that a previous study showed that every 5-centimeter increase in waist circumference was associated with 17% relative increase in the risk of all-cause mortality in men and 13% greater risk in women.

In a subset of the participants, the researchers were able to examine the relationship of physical activity and sedentary time with waist circumference in a longitudinal fashion. Neither the amount of time spent exercising nor sedentary time was associated with waist circumference at follow-up.

A higher waist circumference at baseline, however, was associated greater amounts of sedentary time at follow-up.

The authors noted that the observational design of the study precludes conclusions about causality. The study might also be subject to residual confounding.

Funding for this project was provided by the National Prevention Research Initiative, U.K. (NPRI-UK). Funding partners included the British Heart Foundation; Cancer Research U.K.; Department of Health; Diabetes U.K.; Economic and Social Research Council; Medical Research Council; Research and Development Office for the Northern Ireland Health and Social Services; Chief Scientist Office, Scottish Executive Health Department; Stroke Association; Welsh Assembly Government; and World Cancer Research Fund.

Ekelund and two co-authors reported links with NPRI-UK. One of the study authors reported being the founder of Esliger Consulting, which produces KineSoft, the analytical software used to generate the accelerometry outcome variables in this study.

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